bnxt_en: improve interrupt coalescing defaults and sysctl handling

[chandrakanth.patil_broadcom.com]

Summary

Enhance interrupt coalescing support by improving defaults and sysctl behavior:

• Initialize Rx and Tx coalescing parameters with sane defaults.
• Set defaults early in bnxt_attach_pre() to avoid divide-by-zero faults when sysctls are queried before the interface is brought up.
• Simplify sysctl handling for Rx and Tx completion coalescing.
• Defer HWRM coalescing commands until the interface is UP, preventing failures when sysctls are set with the interface down.
• Add a sysctl tunable for coalescing statistics collection interval, with default changed to 250ms.